Mr. Crowley is chairman of the Democratic caucus, making him the fourth-ranking Democrat in the House. He also was the heir apparent to the minority leader, Nancy Pelosi. Ms. Pelosi, now 78, has herded Democratic cats for nearly 16 years and has made clear her desire to stay put if the party wins back the majority in November. But she’s expected to face a serious challenge to her rule after the election, whether or not the party reclaims the majority. A swelling segment of Ms. Pelosi’s caucus — especially younger lawmakers — feel it is time for a change, and at least 20 Democratic candidates have pledged not to support her for leader if they win their races or have called for a change in leadership.

But with the loss of Mr. Crowley, who was a relative young ’un in the Democratic leadership, at age 56, House Democrats have nothing even approaching a backup plan. This has set off a fresh round of hand-wringing and squabbling — privately, if not publicly — about when a new generation will get a chance to lead, and who should be the face of that leadership. This is hardly a useful distraction during a high-stakes election year. But it’s a situation for which Ms. Pelosi has only herself to blame.

For too long, this regime has clung to power at the expense of future leaders. Neither of Ms. Pelosi’s two chief deputies, Steny Hoyer, the party whip, and Jim Clyburn, the assistant leader, is a remotely viable successor. Like Ms. Pelosi, both men are just shy of 80 and have occupied top-tier posts for over a decade. At this point, the caucus leadership has gone from stale to downright ossified.

Of course, experience and maturity can be valuable traits. But Democrats are supposed to be the party of the future, the forward-looking party, the party of youth and progress. That spirit is hard to convey with a House leadership team that looks uniformly and dramatically like the past. The optics are particularly jarring in contrast to the lineup of Republican leaders, who are all in their late 40s (Speaker Paul Ryan) or early 50s (the majority leader, Kevin McCarthy, and the chief whip, Steve Scalise).

The Democrats’ failure to aggressively nurture fresh talent has had a concrete impact on the caucus. Promising up-and-comers have long found themselves languishing, with nowhere to up and come. The lawmakers most often discussed as potential leaders in recent years — Xavier Becerra, the California attorney general; Chris Van Hollen, the senator from Maryland; and Steve Israel, the former congressman from New York — ultimately chose to leave the House rather than wait around indefinitely for an opportunity to advance. Frustrated members fret that by refusing to make room for fresh blood, leadership is damaging the caucus’s long-term health.

This is not merely a question of who holds the top three jobs. Plum committee assignments are doled out through a combination of seniority and leadership’s blessing. And once people settle into a prime post, they dig in. On the Republican side, by contrast, committee leadership is term-limited. This presents its own set of issues, but it does provide for greater churn and more opportunities for junior members.

On occasion, a younger Democrat will poke at the problem. In the wake of the party’s devastating election loss in 2016, as irritation with Ms. Pelosi spiked, Representative Tim Ryan of Ohio launched a challenge to the leader. Characterized as a suicide mission by his colleagues, Mr. Ryan’s bid was never expected to succeed. Rather, it was meant to signal the extreme twitchiness in the ranks. To calm the waters, Ms. Pelosi agreed to create new leadership opportunities, such as directing each committee to establish a vice chair/vice ranking member seat specifically reserved for junior members. Even so, frustration continues to bubble up.

Most notably, last October, Linda Sánchez, the caucus’s vice chairwoman, sent a jolt through the caucus when she publicly called for the current regime to “pass the torch.” It was a rare and risky display of disloyalty from within Ms. Pelosi’s own leadership team, prompting much chatter about whether Ms. Sánchez would lose her leadership spot in the next Congress. Ms. Pelosi has a reputation for making life unpleasant for those who cross her.

While Mr. Crowley’s loss may have come as a shock to everyone, the leadership disarray it has caused was predictable. Because of this regime’s failure to foster new talent, the caucus lacks a deep bench — or any bench, really — of members ready to assume Ms. Pelosi’s mantle. At this point, some in the caucus are toying with the idea of using Mr. Hoyer as a temporary placeholder for a couple of years until a younger replacement can be properly groomed for the job.

Such a stopgap measure, obviously, is not the kind of fresh approach and generational change that many of the House Democrats have been hoping for. But this is what happens when leaders focus more on maintaining their own power than on preparing their party for the future.